HDAC is a generic term for proteins deacetylating histone, and mainly controls gene expression in the nucleus of cells. HDAC has various types, and is reported to be deeply related to pathological conditions such as immune, inflammation, cancer, nervous disease and the like. The gene expression regulation by HDAC is dependent on kinds of cell, target protein to be acted on, or cellular environment (Non-Patent Document 1).
Acetylation of histone is one of important determinants for gene expression. It is known that acetylation of histone generally acts in the direction of acceleration of transcription, and deacetylation of histone generally acts in the direction of suppression of gene expression. HDAC is a generic term for enzymes removing an acetyl group from lysine residue of target protein including histone. HDAC family is classified into four kinds of HDACs (class I HDACs (HDAC1, 2, 3, 8), class II HDACs (HDAC4, 5, 6, 7, 9, 10), class III HDACs (SIRT1-7), class IV HDAC (HDAC11)). Among them, class I HDACs is ubiquitously expressed, and mainly localized in the nucleus. It shows high enzyme activity against histone, and its role as modification of histone and transcription repressor is widely studied. Class II HDAC is classified into IIa (HDAC4, 5, 7, 9) and IIb (HDAC6, 10) based on the domain structure. Class IIa HDACs have an N-terminal domain bonded to transcription factor and a C-terminal domain having a nuclear transport signal, and can move between nucleus and cytoplasm. Unlike the other HDACs, its expression pattern is comparatively localized. For example, HDAC5 and HDAC9 are expressed in muscle, heart and brain. On the other hand, class IIb HDACs has a tandem structure of deacetylating domain, unlike class IIa HDACs, and HDAC6 is mainly expressed in cytoplasm. As the target molecule of HDAC6, α-tubulin and cortactin and the like, which are cytoskeleton proteins, are reported. It is known that low molecular HDAC inhibitors cause various cellular reactions such as cell-growth inhibition, cellular differentiation and cellular apoptosis, and HDAC inhibitors such as SAHA (vorinostat) and FK228 (romidepsin) are presently clinically used for T-cell malignant lymphoma as indication. In addition, effects of HDAC inhibitor on animal models of some inflammatory diseases, for example, models of arthritis, inflammatory bowel disease, GvHD, sepsis and the like are also reported (Non-Patent Documents 1, 2 and 3).
It is reported that vorinostat and trichostatin, which are HDAC inhibitors, show symptom improvement of pathological condition and actions such as protection action and the like in various animal models of autoimmune disease or inflammatory disease including arthritis model, enteritis model, GvHD model and the like (see Non-Patent Documents 4 to 7). In addition, it is reported that tubacin, which is a HDAC6 inhibitor, enhances regulatory T cell inhibitory action, and suppresses T-cell-dependent immune response in experimental enteritis model (Non-Patent Document 8). Therefore, HDAC inhibitor and/or HDAC6 inhibitor can be therapeutic drugs for various autoimmune diseases and/or inflammatory diseases such as inflammatory bowel disease, rheumatoid arthritis, psoriasis, Sjogren's syndrome, Behcet's disease, multiple sclerosis, systemic lupus erythematosus and the like, GvHD and the like (Non-Patent Documents 2 and 9).
In addition, HDAC has an important role in tumor formation because it regulates activities of tumor suppressor gene and oncogene. For example, it is reported that overexpression of HDAC in prostate cancer, colorectal cancer, breast cancer, lung cancer, liver cancer, stomach cancer and the like correlates with decrease in disease-free survival and overall survival (Non-Patent Document 3). Therefore, HDAC inhibitor targeting solid cancer and blood tumor is developed. Vorinostat and romidepsin, which are HDAC inhibitors, have been approved by FDA as a therapeutic drug for T-cell malignant lymphoma, and plural HDAC inhibitors are preclinical or in clinical trials (Non-Patent Document 10). In addition, it is reported that ACY-1215, which is a HDAC6 inhibitor, has a tumor growth inhibitory action or an extended survival action in multiple myeloma model, when used in combination with bortezomib (Non-Patent Document 11). Therefore, HDAC inhibitor and/or HDAC6 inhibitor can be therapeutic drugs for cancers such as multiple myeloma, leukemia, uterine leiomyosarcoma, prostate cancer, cachexia, myelofibrosis and the like.
On the other hand, it is reported that vorinostat and valproic acid, which are HDAC inhibitors, show actions such as improvement of spatial memory, increased motor function and the like in animal models such as Alzheimer's disease model, Huntington's disease model and the like (Non-Patent Document 12). In addition, it is reported that ACY-738 and ACY-775, which are HDAC6 inhibitors, show a significant antidepressant action in ethopharmacological experimants such as tail suspension test and the like (Non-Patent Document 13). Moreover, it is reported that HDAC6 also has an important role in regulation of amyloid β involved in maintenance of homeostasis of tau and stability of microtubule which are deeply related to Alzheimer's disease, and that inhibition of HDAC6 improves memory in neurodegeneration mouse model in water maze test using HDAC6 knockout mouse and APPPS1-21 mouse which is a Alzheimer's disease mouse model (Non-Patent Documents 14 and 15). Therefore, HDAC inhibitor and/or HDAC6 inhibitor can be therapeutic drugs for central nervous system diseases including neurodegenerative diseases.
The compounds having a structure similar to that of the compound described in the present specification are, for example, the following compounds.    (1) Patent Document 1 discloses a compound represented by the following formula:
wherein each symbol is as defined in the document, which is a class II HDAC (HDAC4, HDAC5, HDAC6, HDAC7, HDAC9) inhibitor, and useful for the treatment of autoimmune disease, immune disease, inflammatory disease and the like.    (2) Patent Document 2 discloses a compound represented by the following formula:
wherein each symbol is as defined in the document, which is a class II HDAC (HDAC4, HDAC5, HDAC6, HDAC7, HDAC9) inhibitor, and useful for the treatment of autoimmune disease, immune disease, inflammatory disease and the like.